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    <title>Document</title>
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<body>
    <script>
        // 作用域链：函数使用变量的时候，采用最近原则。

        // 案例1：结果是几？
        var num = 456;

        function f1() {
            var num = 123;

            function f2() {
                var num = 0;
                console.log(num); //站在目标出发，一层一层的往外找
            }
            f2();
        }
        f1();

        // 案例2 ：结果是几？
        var a = 1;

        function fn1() {
            var a = 2;
            var b = '22';
            fn2();

            function fn2() {
                var a = 3;
                fn3();

                function fn3() {
                    var a = 4;
                    console.log(a); //a的值 ?
                    console.log(b); //b的值 ?
                }
            }
        }
        fn1();
    </script>
</body>

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